Prior to the present invention the prior art has been interested in obtaining pigments endowed with dulling properties, of photostability and in general with pigmentary characteristics comparable with those of conventional inorganic pigments such as chromium yellows, cadmium yellows, the preparation and use of which always meet with increasingly severe limitations because of their toxicity.
In an attempt to achieve the above-mentioned objectives, the prior art has suggested a series of solutions based on the attribution and/or enhancing of the pigmentary properties in organic dyes of an acid type, by means of lacquering and supporting technologies that may lead to complexes of a pigmentary nature consisting of an organic and an inorganic portion.
In this connection it is known that the acid dyes are compounds that contain in the molecule acid groups (COOH, SO.sub.3 H) which impart to the products a solubility in water or in alkaline media, and are mainly used in the dyeing of either natural or synthetic polyamidic fibres. It is likewise known that some of these compounds may form insoluble compounds (`toners`), lacquers, in combination with alkaline-earth metal salts (Ca, Ba, Mg, Sr, Al) and/or of another nature (Mn, Zn).
These latter, like most part of the organical pigments, do not possess the dulling properties (hiding power) or traditional inorganic pigments (chromium yellows, cadmium yellows). On the contrary, in the dyeing stage of the manufactured article, they are mechanically associated with inorganic fillers of various natures. Depending on the type of application, such charges may consist of compounds (for instance barites) of modest hiding power or compounds (for instance TiO.sub.2) of high hiding power.
Nevertheless, in the case of fillers with a high hiding power (TiO.sub.2), there is required the use of highly colored compounds and/or in a great quantity in order to achieve high intensities of coloring, which consequently involves considerable economical burdens.
Also, the prior art reports the association of the organic pigment with inorganic fillers, carried out in the stage of the sythesis of the pigment itself. In fact, there are known products of the lacquer type, which are prepared in the presence of inorganic supports amongst which the most common are basic Al compounds (basic sulphate-Pigment White 24) or mixtures of BaSO.sub.4 and aluminum hydrate (Pigment White 23).
According to above said technique, the precipitation of the lacquers occurs normally in two stages. For example, at the start one precipitates, under controlled pH- and temperature conditions, from Al sulphate the basic aluminim sulphate. Then, in the stated order of addition, there are added the solution of the dye and the solution containing a metered quantity of BaCl.sub.2 in such a way as to cause the simultaneous precipitation of the insoluble salt of the organic dye and of the Barium sulphate on the Al(OH).sub.3 substrate. This technique allows, substantially, the preparation `in situ` (on the spot) of the inorganic substrate and allows one to obtain a dye-receptive surface but with poor hiding power.
According to other known processes, in the synthesis stage, there are used inorganic substrates of high hiding powers (TiO.sub.2). In fact, it is known that acid dyes may be precipitated under controlled pH conditions (pH=2-5) in the presence of TiO.sub.2 and other organic matrixes.
In the above-referred to prior art approaches, the aforementioned prior art provides no appreciable advantages in comparison with the mechanical mixtures of the single components.
According to another prior art technique, the acid dye is precipitated, in the form of Al lacquer, Cr lacquer, etc., in the presence of lignisulphonates.
Common characteristics of these co-precipitation processes is that of obtaining, in any, the formation of organic-inorganic compositions, substantially consisting, on a binary base, of a lacquer of an organic dye supported by precipitation on an inorganic matrix.